Hydropneumatic accumulator

ABSTRACT

This invention relates to a hydropneumatic accumulator comprising a piston and cylinder arrangement defining in the cylinder a first compartment in which is provided a gas which is under pressure and which has a density less than that of nitrogen and a second compartment in which liquid is provided. Helium is the gas preferably used, such that, on the one hand, a much larger volume of the liquid can be stored in the accumulator at a predetermined pressure than is possible using nitrogen, and, on the other hand, the variations in pressure due to variations in temperature is much less than with nitrogen. The invention is particularly applicable to hydraulic controls for electric circuit breakers.

United States Patent [191 Gratzmuller Dec. 24, 1974 HYDROPNEUMATICACCUMULATOR [76] Inventor: Jean Louis Gratzmuller, 66

Boulevard Maurice Barres, Hauts-de-Seine, France Related US. ApplicationData [63] Continuation of Ser. No. 110,951, Jan. 29, 1971,

abandoned.

[30] Foreign Application Priority Data [58] FieldofSearch..138/30,31;267/113,118, 267/122, 124

[56] References Cited UNITED STATES PATENTS 2,170,890 8/1939 Allen138/31 1 IITIIIIIHHIII 2,747,370 5/1956 Traut 138/31 2,829,672 4/1958Bleasdale... 138/31 2,999,680 9/1961 Eiseman, Jr... 267/64 R 3,064,68611/1962 Gratzmuller 138/31 3,326,241 6/1967 Mercier 138/30 PrimaryExaminer-Charles A. Ruehl Attorney, Agent, or Firm-Lilling & Siege] [57]ABSTRACT This invention relates to a hydropneumatic accumulatorcomprising a piston and cylinder arrangement defining in the cylinder afirst compartment in which is provided a gas which is under pressure andwhich has a density less than that of nitrogen and a second compartmentin which liquid is provided. Helium is the gas preferably used, suchthat, on the one hand, a much larger volumeof the liquid can be storedin the accumulator at a predetermined pressure than is possible usingnitrogen, and, on the other hand, the variations in pressure due tovariations in temperature is much less than with nitrogen. The inventionis particularly applicable to hydraulic controls for electric circuitbreakers.

9 Claims, 6 Drawing Figures FATENTED DEC 24 1974 saw 3 m 5 AmEOV ZOUEDJO wmnwwwmm PATENTED UEC24 I974 sum u (5 g PRESSURE DECREASE FIG. 4

HYDROPNEUMATIC ACCUMULATOR This is a continuation of application Ser.No. 110,951, filed Jan. 29, 1971, now abandoned.

BACKGROUND OF THE INVENTION It is known that hydropneumatic pistonaccumulators are substantially made up of a sealed cylinder divided by apiston into two compartments of volumes which are inversely variable,one compartment enclosing a cushion of gas under pressure, forming anelastic buffer, the other compartment containing a liquid, generallyoil, which is thus stored and always available at the pressureestablished by the cushion of gas.

Such accumulators are widely used in hydraulic control installations(e.g., hydraulic circuit breaker controls). where they ensure that thereis always available a predetermined minimum volume of oil at apredetermined minimum pressure. In these installations, the accumulatorsare generally periodically recharged or reinflated with oil by a pump.

The gas used as an elastic cushion in conventional accumulators wasoriginally air, but the oxidizing effects of the oxygen content of theair were sometimes damaging under high pressure, so that, for a numberof years, nitrogen has been almost universally used in preference toair, as it is inert and very cheap. In practice, the large proportion ofnitrogen in the air allows the characteristics of air-inflatedaccumulators to be compared to those of nitrogen-inflated accumulators,so that the following remarks apply equally to both gases. 1

Until the present day, the pressures frequently used in hydrauliccontrol plant comprising oil-pneumatic accumulators were of the order ofabout 100 to 300 kg/cm and accumulators using nitrogen weresatisfactory.

However, certain applications now require working pressures above 300kg/cm and, for example, of the order of 100 to 700 kg/cm and as much as1,000 kg/cm It will be these pressures, above 300 kg/cm which will bedesignated high pressures" in the followmg.

It was proved that, for these high pressures, and even starting from 300kg/cm conventional nitrogen accumulators had certain drawbacks, and itwas necessary to increase their dimensions, and consequently theirprice, to a substantial degree if it was desired to have available avolume of oil under pressure which was substantially unchanged inrelation to medium-pressure accumulators. Again, high temperatures (e.g.of 50C, which may be encountered in outdoor installations exposed to thesun) proved to be very disadvantageous in high-pressure nitrogenaccumulators, causing a loss in available energy which is proportionallymuch greater than in the case of medium pressures.

For the sake of simplicity it may be stated that the difficultiesarising at high pressures with conventional nitrogen accumulators aredue to the loss of compressibility of the nitrogen in proportion as thepressures increase. Consequently, the nitrogen fulfils its function ofan elastic cushion less and less efficiently and tends to behaveprogressively as a liquid as the pressures rise. At pressures between200 and 300 kg/cm the loss of compressibility of the nitrogen is alreadyof the order of percent.

In fact Mariottes (Boyle s) law PV-C" is only a limit law, applicable tothe perfect gaseous state, a state more closely approached by knowngases in relation as they move away from their critical point. In thecase of high-pressure accumulators, used at ambient temperatures between40 and 9509C, the physical properties of nitrogen move considerably awayfrom those of a perfect gas, resulting in a serious deterioration in theelastic qualities of the gas.

It is an object of the present invention to obviate or mitigatedrawbacks such as outlined above and allow hydropneumatic high-pressureaccumulators to be produced which are capable of storing and replacingmore energy (e.g. 50 percent more energy) than a conventional nitrogenaccumulator of the same dimensions and functioning at the same pressure.

The present invention is a hydropneumatic accumulator comprising asealed cylinder having a piston slidably located thereon and definingwith the cylinder a first compartment enclosing a gas under pressure anda second compartment enclosing a liquid, the improvement that the gasenclosed in said first compartment is a gas of a density less than thatof nitrogen.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the present inventionwill now be described by way of example with reference to theaccompanying drawings, in which:

FIG. 1 is a diagrammatic view of an accumulator according to theinvention in an hydraulic control installation;

FIG. 2 is a graph showing the volumes of oil available as a function ofthe pressures in an accumulator according to the invention, and in aconventional nitrogen accumulator at a temperature of 20C.

FIG. 3 is an analogous graph, but drawn for temperatures of +50 and 35C;

F IG. 4 is a diagrammatic view in section of an electrical circuitbreaker fitted with a static hydropneumatic accumulator according to theinvention;

FIG. 5 is a diagram showing the advantages of accumulators according tothe invention for static applications; and

FIG. 6 shows an embodiment of a hydropneumatic accumulator according tothe invention and, diagrammatically, the hydraulic installation on whichthis accumulator is mounted.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 illustrates ahydropneumatic accumulator which comprises a sealed container formed bya cylinder 3 and two end parts 5 and 7. The cylinder 3 has a piston 13slidably located therein, the piston 13 defining with the cylinder 3,two compartments 9 and 11. Compartment 9 encloses a gas under pressurei.e., according to the invention a gas of a density less than that ofnitrogen, such as helium, hydrogen or neon, while compartment 11contains oil.

It has proved that such gases reveal a loss of compressibility at thehigh pressures envisaged which is much lower than that encountered withnitrogen in conventional accumulators, such gases being moresimilar toperfect gases than nitrogen in the conditions of operation envisaged.

Again, helium is preferable chosen, as it is not flammable, ischemically neutral and is easily obtained. It will be shown later that,in spite of the high cost of helium compared to nitrogen (about 15 timesdearer), an

accumulator inflated with helium is less costly than one inflated withnitrogen, for identical performances.

The construction of such an accumulator is conventional, and needs nofurther detailed description.

FIG. 1 also shows the essential elements of a conventional workingcircuit of such an accumulator. The oil compartment 11 is connected by apipe 15 to a motor apparatus such as a jack 17, a rod 19 of which canactivate any controllable device e.g. the mobile contact of anelectrical circuit-breaker, if the installation is used as an hydrauliccircuit-breaker control. A valve 21 interposed in the'pipe 15 allows thejack 17 to be selectively linked to the high pressure of the accumulatoror to a low-pressure tank or container 23. A pump 25 draws oil from tank23 and recharges compartment 11 of the accumulator with oil underpressure. It may thus be said that in this application, the accumulatoroperates as an active appliance, receiving or restoring energy.

A pipe 27, equipped with a stop-valve 29 allows compartment 9 of theaccumulator to be supplied with gas at the initial pre-inflationpressure, and also allows the accumulator to be recharged if there is agas leak.

There will now be a brief survey of the method of operation ofhydropneumatic accumulators in order to bring out better the advantagesattained by the invention. If for example it is desired to provide anaccumulator capable of delivering oil at a pressure between 400 and 600kg/cm the compartment 11 is empty or almost empty of oil, i.e., thepiston 13 is at that moment against or in the vicinity of the lower endpart 7 of cylinder 3, and oil is introduced into compartment 11 by meansof pump 25, this oil pushing back piston 13 and compressing the gaseouscushion 9.

After introduction of a determined volume V of oil, the pressure reachesthe maximum pressure selected, e.g., 600 kg/cm and the pump stops. Thusfrom this point there is in the accumulator a reserve of energy, readilyavailable, corresponding to the outlet of a volume V of oil at apressure between 600 and 400 kg/cm In practice, of course, variation inambient temperature must be taken into account, as this will cause thepressures to vary, and the above indications have been given for reasonsof simplification, it being assumed that the operating temperature isconstant.

FIG. 2 shows, in the form of a graph, the functioning of an accumulatoras described above. The volumes of oil in cm introduced into theaccumulator, or capable of being restored by it, are entered asabscissae, and the corresponding pressures in kg/cm as ordinates.

Curve A relates to a conventional accumulator charged with nitrogen,curve B to an accumulator according to the invention charged withhelium, and curve C is a theoretical curve representing the functioningof an accumulator charged with a perfect gas which obeys exactly, forthe high pressure considered, Mariottes (Boyles) law PVC". These threecurves are drawn for an identical temperature of +20C and for anidentical accumulator of a total internal capacity of 1,000 cm, capableof delivering oil at a pressure between about 400 and 600 kg/cmConsidering curve A first, the abscissa shows that the accumulator waspre-inflated with nitrogen at a pressure of about 400 kg/cm Oil is thenintroduced under pressure into the oil compartment. Point 33 on thecurve shows that, after introduction of 123.5cm of oil the pressure ofthe accumulator is raised to about 500 kg/cm and, after introduction of206cm of oil (point 35 of the curve) the pressure reaches about 600kg/cm In short, with such a conventional nitrogen accumulator, there isprovided a reserve of energy constituted by the output of 206cm of oilat a pressure between about 600 and 400 kg/cm (at a temperature of 20C).It should be noted here that the volumes indicated on the curves arethose actually occupied by the oil at the pressure under consideration;in fact, at these high pressures, the oil is relatively compressible(about 3/100 for about ,600 kg/cm Curve B will now be examined relativeto an accumulator of identical construction and capacity, but chargedaccording to the invention with a gas less dense than nitrogen, heliumin the example chosen (density of helium relative to air; 0.137; densityof nitrogen relative to air; 0.97).

The origin of curve B is identical to that of curve A, both accumulatorsbeing pre-inflated to the same initial pressure of about 400 kg/cm It isseen at point 37 on curve B that 180.5cm (instead of l23.5) of oilintroduced reaches a pressure of about 500 kg/cm The final pressure ofabout 600 kg/cm (point 38) is reached after introduction of 303 cm ofoil. The result is that with such a helium accumulator a reserve ofenergy is available in the form of an outlet of oil of 303 cm at apressure between about 600 and 400 kg/cm instead of only 206 cm underthe same conditions with a conventional nitrogen accumulator. The energygain is thus in the region of 50 percent, with the bettercompressibility of helium compared to nitrogen, at the high pressuresunder consideration.

The theoretical curve C, representing the phenomenon if the accumulatorcould be charged with a perfect gas (PV-C) has above all the object ofshowing the loss of compressibility which arises with nitrogen, thisloss being much less with helium (or with another less dense gas such ashydrogen). The point 39 on curve C corresponds to a pressure of about600 kg/cm and to a volume of oil of 333 cm It may be deduced from thisthat the nitrogen accumulator allows only 206/333 62 percent of themaximum theoretically storable energy to be stored, while the heliumaccumulator allows 303/333 91 percent of this maximum theoreticalenergy.

A gas less dense than nitrogen, which is likewise suitable for anaccumulator according to the invention is hydrogen (density relative toair; 0.069), whose curve (not shown) would be located substantiallybetween those for nitrogen and helium. Point 40 appearing in Flg. 2, fora pressure of about 600 kg/cm corresponds to a volume of available oilof about 255 cm, in an accumulator inflated with hydrogen, i.e. anincrease in available energy of 24 percent relative to a conventionalnitrogen accumulator.

Finally, neon (density relative to air; 0.674) could also be usedadvantageously in an accumulator according to the invention.

As a real gas becomes more similar to a perfect gas in proportion as itmoves away from conditions corresponding to its critical point, it is ofinterest to examine the critical constants of the different gasesconsidered above. These constants are indicated in the following tableto which has been added the density of the gas relative to air at aboutnormal conditions.

critical critical density Temp. C pressure kg/cm helium -268 about 2.250.137 hydrogen 240 about 12.8 0.069 neon 205 about 29.0 0.674 nitrogenl47.l about 33.5 0.967

This clearly shows that the gases selected, particularly helium, will bemuch further removed from their critical point at the high pressuresenvisaged and for the current ambient temperatures (40C to 50C) hencethe gain in compressibility.

It may also be noted that helium, despite its cost, which is about timeshigher than that of nitrogen, allows hydropneumatic accumulators to bemade more economically than those filled with nitrogen. In fact, if thesame available reserve of energy were required, the total internalcapacity (and thus the dimensions) of a conventional nitrogenaccumulator would have to be about 50 percent greater than those of ahelium accumulator according to the invention, which would increase thecost price by about 50 percent, while the increase in cost price due tothe use of helium in the place of nitrogen is less than 7 percent.

These conclusions are valid for the isothermal functioning of theaccumulator, at a constant ambient temperature but, as has beenindicated before, the performances of hydropneumatic accumulators aremodified as a function of ambient temperatures.

In FIG. 3, curves D and E are the function curves respectively at 35Cand +50C of a helium accumulator according to the invention, havingserved to trace the curve B in FIG. 2. Curves F and G are the curves,respectively at 35C and +50C of a conventional nitrogen accumulator ofthe same capacity, having served to trace the curve A in FIG. 2.

In both cases, the accumulator was initially preinflated to a pressureof 400 kg/cm at a temperature of C, as in the case of FIG. 2, and thetotal interior capacity of the accumulator is 1,000 cm, as indicatedbefore.

Naturally, variations in temperature cause the preinflation pressures tovary (pressures initially established, e.g. at 20cm when the accumulatoris empty of oil), as well as the quantities of oil available. Thesevariations, visible on the curves in FIGS. 2 and 3, are reproduced inthe following table:

variation in pre-inflation in kg/cm as a function Quantity of oil in cmVolume of oil output Helium Nitrogen from 600 kg/cm accumulatoraccumulator 135 cm 482 kg/cm 408 kg/cm 230 cm 422 kg/cm It is seen, atthis temperature of 35C that not only is the advantage of 70 percent inoutput of available oil 'which existed at +C retained, but that thepressures are between 600 and 408 kg/cm for Nitrogen, whereas they arebetween 600 and 422 kg/cm (still with the greater output) for helium.The gain in available energy is thus again improved.

The result of the above is that, with an accumulator according to theinvention, inflated with helium, the following advantages are attainedcompared to a conventional accumulator inflated with nitrogen.

1. a gain in quality, stemming from the reduced pressure drops;

2. a gain in available energy of at least 50 percent when functioning ata constant ambient temperature;

3. a gain in available energy of at least 70 percent when operating atvery variable ambient temperatures, and for example, in a range between+50C and 35C;

4. A cost increase which does not exceed about 7%,

for a minimum energy gain of 50 percent.

Finally, it may be said that, even when operating at a constant ambienttemperature, the expansion is never absolutely isothermal, so that theincrease in energy produced is even greater than that indicated above,for the drops in pressure are less in reality.

With reference to FIGS. 4 and 5, there will now be described theapplication of a helium accumulator acof temperature contained in theaccumulator at 600 kg/cm 35C +20".C +50C 35C +20C +50C nitrogen 278 400471 358 206 135 helium 330 400 449 445 Thus it is seen that anaccumulator of a total internal capacity of 1,000cm, pre-inflated to 400kg/cm at 20C, and whichwould be used in a temperature range cording tothe invention to static work consisting in keeping a fluid at apredetermined pressure and particularly, in keeping in the liquid statea liquefiable dielectric gas serving to insulate a circuit-breaker.

The circuit-breaker shown in FIG. 4 has already been described andillustrated in French Patent 1,430,333. It is enough to recall that acut-off chamber 52, in which a fixed contact 62 is mounted, and in whicha mobile contact 56, actuated by an hydraulic jack 66, is filled with aliqueflable dielectric gas, e.g., SF6 or Freon. This dielectric gas iskept permanently in the liquid state by the pressure exerted by a pistonaccumulator 78.

The accumulator 78 comprises a cylinder 94 in which is slidably locateda piston 80 defining with the cylinder 94 a gas compartment 82, filledwith a gas less dense than nitrogen, particularly helium, underpressure, and a liquid compartment 76 filled with the above mentioneddielectric gas in the liquid state. The liquid compartment 76communicates with the internal volume 69 of the cut-off chamber 52 by anarrow-bore tube 74.

A valve 84 allows the accumulator to be reinflated and, by means of amonometer 86, the maintenance of the pressure of the dielectric fluid inthe circuit-breaker can be checked. In the above mentioned Patent, itwas indicated that nitrogen was chosen to fill the gas compartment 82,i.e. to constitute the cushion of elastic gas of the accumulator. It wasalso indicated that the manometer 86 could be used to automaticallycontrol operations for re-establishing the pressure.

According to the present invention, nitrogen is not used to form thegaseous elastic cushion, but a gas less dense than nitrogen, and inparticular, helium. It will be shown in the following that, due to thehelium, the pressure limits of the dielectric fluid necessary forcorrect functioning of the circuit-breaker can be naturally respectedwithout having recourse to an automatic system for controlling andregulating the pressure, as might be necessary with an accumulatorinflated with nitrogen.

As indicated above, the variations in temperature accepted in France forcircuit-breakers are from --35C to 50C.

Moreover, in the case of a liquefied dielectric gas (SP6)circuit-breaker, in the closed condition of the circuit-breaker, thepassage of the current may raise the temperature of the SP6 to 30C.

Consequently, the extreme temperatures for the liquid may vary from 35C(cold, open) to +80C (hot, closed) i.e. a temperature differential ofthe liquid of 115C. Of course, this differential of 115C will onlyaffect the liquid contained in the cut-off chamber, whereas theaccumulator itself will only be subjected to variations in externaltemperature i.e. from 35C to +50C.

The variations in pre-inflation pressures have been indicated above as afunction of temperature, in the case of a nitrogen accumulator, and inthat of a helium accumulator, both pre-inflated to 400 kg/cm at atemperature of 20C.

It is sufficient to recall that the said variations are from 278 to 431kg/cm for nitrogen, and from 330 to 449 kg/cm for helium, between -35Cand +50C.

It is very evident that when a circuit-breaker of the type shown in FIG.4 is set up, methods must be provided to keep the pressure in thedielectric liquid between predetermined limits, starting from a meanpressure, despite the variations in temperature.

The variations are mainly due to the expansion of the gas and also tothe expansion of the liquid in the cut-off chamber.

One of the known methods consists in using an accumulator of very largevolume i.e. with a large-volume gas compartment. Thus, if it is assumedthat the variations in volume of the liquid due to temperature fluctuations cause a volume variation of in the gas compartment, thecorresponding pressure variations would be of the order of 10%. But sucha method would lead to the use of accumulators of very large dimensions,which would be unnecessarily expensive, and would only compensate forthe expansion of the liquid, to the exclusion of that of the gas.

It is seen from now on, however, that the use of a helium accumulatoraccording to the invention will reliably allow limitation of thepressure variation due to gas expansion to 449 330 l 19 kg/cm whereas,with a conventional, nitrogen accumulator, they would be 471 278 193kg/Cm Considering the case of a circuit-breaker in which the dielectricis liquid SP6, the liquid has a coefficient of expansion is 4 10 perdegree C, which, for a temperature differential of l 15C, results in avolume variation of the liquid of 4.5 percent. The compressibility ofliquid SP6 is only 10 per kglcm and is thus relatively insignificant. Itwill not be taken into account in the following.

It will be assumed that an accumulator of a volume of 1,00Ocm isselected, as in the example quoted above.

It will also be assumed that, for a determined installation, a pressureof 600 kg/cm is fixed as a maximum acceptable hot pressure, both with anitrogen and with a helium accumulator.

Under these conditions, at the minimum temperature, and with constantvolume, the pressure will drop to 430 kg/cm in the case of helium (i.e.the decrease in pressure would be 28 percent), while it will drop to 339kg/cm in the case of nitrogen pressure decrease 43 percent.

These results are illustrated in FIG. 5, where the percentages ofpressure decrease are shown horizontally, based on the maximumacceptable pressure when the temperature decreases.

According to this PIG., may be made:

A. It will be impossible with any accumulator of the volume selected torespect a pressure decrease below 28 percent;

B. If the acceptable pressure decrease is between 28 and 43 percent, ahelium accumulator of a sufficiently high volume according to theinvention will fulfil the requirements set, and will thus ensure, in astatic manner and without any regulation apparatus, that the pressure iskept between the selected limits.

On the contrary, no conventional nitrogen accumulator, whatever itsvolume, will be able to satisfy such Conditions.

C. The nitrogen will only be able to maintain the pressure for decreasesabove 43 percent.

To summarise, the nitrogen accumulator will only be suitable in zone Cof FIG. 5. The helium accumulator will be suitable in zones B and C. Noaccumulator will satisfy the requirements of zone A.

By means of an experimental example, the advantage of the accumulatoraccording to the invention will be illustrated, in comparison withconventional accumulators. A circuit-breaker will be postulated, inwhich the volume of dielectric liquid (SP6) is such that the volumevariations (hot and cold) of the liquid are 1,000 cm, i.e. theaccumulator must be able to absorb or restore 1,000 cm of liquid.

the following observation A pressure-decrease latitude ratio of 50percent is chosen (e.g. pressure between 600 and 300 kg/cm so that thispressure can be attained by both types of accumulator (Zone C, FIG.

The experiments results show that a helium accumulator of a totalcapacity of 3.5 litres is sufficient, whereas the conventional nitrogenaccumulator, giving the same latitude of performance, should be of l 1.2litres, or more than 3 times greater.

With reference to FIG. 6, there will now be described a hydro-pneumaticaccumulator according to the invention, provided with reference meansindicating the position of the piston in the cylinder of theaccumulator, these means enabling control of the filling of theaccumulator with oil as a function of the volume of oil in the latter,and not as a function of pressure.

A cylinder 103 of an accumulator 101 has a piston 113 slidably locatedtherein, the piston 113 defining with the cylinder 103 a gas compartment109 filled under pressure with a gas less dense than nitrogen, this gaspreferably being helium, and a liquid compartment 111 filed with oilwhich supplies a working circuit 117.

The helium accumulator is provided with an extension rod 127 which isintegral with piston 113, and which passes through a base 105 ofcylinder 103, through a sealed joint 129.

The outer end of the rod 127 preferably having a widened part 131forming a cam, can actuate control keys of one or more electricalcontacts 133-433 incorporated in the supply circuit of an electric motor135 driving an oil filling pump 125.

An accumulator, with a rod which controls electrical contacts, hasalready been described in French Patent 1,181,955.

In the embodiment shown in FIG. 6, contact 133 is a start switch formotor 135, while contact 133 is a stop switch for the same motor. Due tothis arrangement, it is possible to keep permanently in the accumulatora reserve of oil under pressure whose volume is between twopredetermined limits, which are fixed by the position of cam 131 of theemerging rod relative to the keys of the switches 133-133. The positionof cam 131 is preferably adjustable on rod 127, in order to regulate thetripping positions of switches 133-133 There will be indicated by way ofexample in the following the features of a hydropneumatic accumulatorcapable of producing under all circumstances a volume of 1,000 cm ofoil, when the temperatures vary between -35C and +50C.

A. When the oil recharging is controlled as a function of the positionof the piston in the accumulator, e.g. by means of the emerging-rodsystem described above, the total capacity of the accumulator should be3,500 cm in the case of an accumulator filled with helium, and 11,200 cmin the case of a nitrogen accumulator, for a pressure decrease of 50percent. The variations in pressure, for the temperature variationsindicated above, would then be between about 600 and 300 kg/cm, i.e. themaximum decrease in pressure would be 50 percent.

It was seen in the above, with regard to the static" application of theaccumulators, that pressure decreases of less than 28 percent could notbe obtained under the conditions indicated, with any accumulator,whether helium or nitrogen. It was also seen that a nitrogen accumulatorwas unable to ensure pressure decreases of less than 43 percent, whereasthe helium accumulator could itself ensure a pressure regulation between28 and 43 percent, and, of course, beyond. This is why there was chosenin the above example a pressure decrease of 50 percent which isacceptable to both types of accumulator.

From the above example it can be seen that the capacity of the nitrogenaccumulator should be more than three times greater than that of thehelium accumulator, for identical performance. Naturally, with equalcapacity, the helium accumulator would give pressure decreases wellbelow those of the nitrogen accumulator.

B. In the case when the oil recharge is controlled as a function of thepressure in the accumulator, for example by means of manostatic controlof the filling pump, as described in the first application, the pressuredecrease may be reduced much more.

Thus for pressures between about 500 and 600 kg/cm the total capacity ofthe accumulator should be 8.186 cm with a helium accumulator and 15.610cm with a conventional nitrogen accumulator (the preinfiation pressuresbeing respectively about 464 kg/cm and 458 kg/cm at 20C). Here again itis seen that a helium accumulator can have a volume half that of theconventional nitrogen accumulator.

Naturally, the invention is in no way limited to the examples described;it is capable of numerous variations accessible to the specialist,depending on the applications envisaged, and without going beyond thescope of the invention.

Having thus set forth the nature of the invention what I claim hereinis:

l. A hydropneumatic accumulator comprising a sealed cylinder having apiston slidably located therein and defining with the cylinder a firstcompartment and a second compartment, the second compartment enclosing aliquid and the first compartment enclosing a body of gas compressed to amaximum operating pressure of at least 200 kglcm said gas having adensity less than that of nitrogen.

2. An accumulator according to claim 1, in which the gas enclosed in thefirst compartment is helium.

3. An accumulator according to claim 1, in which the gas enclosed in thefirst compartment is hydrogen.

4. An accumulator according to claim 1, in which the gas enclosed in thefirst compartment is neon.

5. An accumulator according to claim 1 in which the gas is compressed toa maximum operating pressure of between 300 and 1,000 kg/cm 6. In anhydropneumatic accumulator comprising a sealed cylinder having a pistonslidably located therein and defining with the cylinder a firstcompartment enclosing a gas under pressure and a second compartmentenclosing a liquid, the improvement that the gas enclosed in the firstcompartment is a gas of a density less than that of nitrogen, and theliquid enclosed in the second compartment is a liquefiable gas kept inthe liquid state by the pressure exerted by the gas contained in thefirst compartment.

7. An accumulator according to claim 6,'in which the gas enclosed in thefirst compartment is helium.

8. An accumulator according to claim 6, in which the liquid enclosed inthe second compartment is a liquefiable dielectric gas kept in theliquid state.

9. An accumulator according to claim 6, in which the liquid enclosed inthe second compartment is liquefied ENTTED STATES PATENT OEETEEQERHHQATE 0F CQRREQTIUN PATENT NO. 3,856,048

D E I December 24, 1974 !NVENTOR(S) 1 Jean Louis Gratzmuller it iscertified that error appears in the ab0veidentified patent and that saidLetters Patent are hereby corrected as shown below:

In 7 Foreign Application Priority Data", change the French applicationnumber "70.03103" to 70,03l05 fiigned and geaied this f f Day 0?Auust1975 [SEAL] G Azresr:

RUTH C. MASON C. MARSHALL DANN Atmsling Officer ('ummissr'mu'r nfl'alcmxand Trademarks

1. A hydropneumatic accumulator comprising a sealed cylinder having apiston slidably located therein and defining with the cylinder a firstcompartment and a second compartment, the second compartment enclosing aliquid and the first compartment enclosing a body of gas compressed to amaximum operating pressure of at least 200 kg/cm2, said gas having adensity less than that of nitrogen.
 2. An accumulator according to claim1, in which the gas enclosed in the first compartment is helium.
 3. Anaccumulator according to claim 1, in which the gas enclosed in the firstcompartment is hydrogen.
 4. An accumulator according to claim 1, inwhich the gas enclosed in the first compartment is neon.
 5. Anaccumulator according to claim 1 in which the gas is compressed to amaximum operating pressure of between 300 and 1, 000 kg/cm2.
 6. In anhydropneumatic accumulator comprising a sealed cylinder having a pistonslidably located therein and defining with the cylinder a firstcompartment enclosing a gas under pressure and a second compartmentenclosing a liquid, the improvement that the gas enclosed in the firstcompartment is a gas of a density less than that of nitrogen, and theliquid enclosed in the second compartment is a liquefiable gas kept inthe liquid state by the pressure exerted by the gas contained in thefirst compartment.
 7. An accumulator according to claim 6, in which thegas enclosed in the first compartment is helium.
 8. An accumulatoraccording to claim 6, in which the liquid enclosed in the secondcompartment is a liquefiable dielectric gas kept in the liquid state. 9.An accumulator according to claim 6, in which the liquid enclosed in thesecond compartment is liquefied hexafluoride of sulphur.